1. Field of the Invention
The present invention relates to a technology for monitoring intensity of a light beam emitted by a light source.
2. Description of the Related Art
Image forming apparatuses capable of performing electrophotographic image recording by using lasers are widely used. Such an image forming apparatus is generally provided with an optical scanning device in which a light beam emitted by a light source is deflected by a deflector (for example, a polygon mirror) to scan the surface of a rotating photosensitive drum in an axial direction, so that a latent image is formed on the photosensitive drum.
However, intensity of the scanning light beam varies with variations in an ambient temperature and aging of the image forming apparatus, causing an image to have uneven density. To prevent such uneven density, the optical scanning devices are generally provided with what is known as auto power control (APC). The APC monitors a portion of the light beam that is emitted by the light source and detected by a light detector such as a photodiode, and controls the output level of a light beam from the light source based on the outcome of the photodiode.
For example, Japanese Patent Application Laid-open No. 2006-91157 discloses an optical scanner that includes a light source, a splitting member, an optical deflecting unit, a second aperture, an optical sensor, a control unit, and a first aperture. The splitting member splits a light beam emitted by the light source into a scanning light beam and a feedback light beam. The optical deflecting unit deflects the scanning light beam and causes the scanning light beam to scan a scanning surface. The second aperture is arranged between the splitting member and the optical deflecting unit and includes a second opening that shapes a cross section of the scanning light beam. The optical sensor receives the feedback light beam and detects the intensity of the feedback light beam. The control unit controls the intensity of the light beam emitted by the light source based on the intensity detected by the optical sensor. The first aperture is arranged between the light source and the optical sensor, and includes a first opening that is larger than the second opening and shapes the cross section of the light beam.
Furthermore, Japanese Patent Application Laid-open No. 2005-156933 discloses an optical scanner that includes an optical system and an intensity control unit. The optical system includes a detecting unit that detects the intensity of an incident light beam and a light splitting unit that splits a portion of the light beam emitted by a light source into a monitoring beam, so that the monitoring beam is made incident on a detecting unit. The intensity control unit controls the intensity of the light beam output by the light source to be of predetermined value. The salient feature of this optical scanner is that if the direction of deflection of the light beam emitted by the light source changes, the optical system performs adjustments so that a rate of change of the intensity on a scanning surface substantially matches with a rate of change of the intensity made incident on the detecting unit.
Moreover, Japanese Patent Application Laid-open No. 2006-259098 discloses an optical scanner that includes a surface emitting laser device, an aperture, a collimator lens, a light deflector, a beam splitting unit, and a light-receiving element. A laser beam emitted by the surface emitting laser device is shaped by the aperture and collimated by the collimator lens. Then, the laser beam is deflected by the light deflector to scan a scanning surface. At the same time, a part of the laser beam is split into a monitoring beam by the beam splitting unit and reflected so that the light-receiving element detects intensity of a reflected light beam. The salient feature of this optical scanner is that the light-receiving element is provided on the same circuit board as the surface light-emitting laser device.
However, an actual angle of divergence of the light beam emitted by the light source usually differs from the angle of divergence at the time of design due to factors such as manufacturing variations in the light sources or rise in temperature in a running optical scanner. Therefore, the intensity variation of the scanning light beam may not be the same as the intensity variation of the monitoring light beam. Thus, accuracy of APC in the optical scanners may be degraded in the technologies described above.